RESPIRATION. 65 



in composition during its stay in the lungs which renders it unfit for further 

 respiration, it becomes requisite, for the correct understanding of respiration, 

 to ascertain the composition of both inspired and expired air. 



Composition of Air. Chemical analysis has shown that every 100 

 vols. of air contains 20.81 vols. of oxygen, and 70.19 vols. of nitrogen, and 

 0.03 vol. of carbonic acid. Aqueous vapor is also present, though the 

 quantity is variable. The higher the temperature the greater the amount. 



The changes in the air effected by respiration are 



Loss of oxygen, to the extent of 5 cubic inches per 100 of air, or i in 20. 

 Gain of carbonic acid, to the extent of 4.66 cubic inches per 100 of 



air or .93 inch in 20. 



Increase of water vapor and organic matter. 

 Elevation of temperature. 

 Increase and at times decrease of nitrogen. 

 Gain of ammonia. 



The total qtiantity of oxygen withdrawn from the air and consumed by 

 the body in 24 hours amounts to 15 cubic feet, and can be readily esti- 

 mated from the amount consumed at each respiration. Assuming that one 

 inch of oxygen remains in the lungs at each respiration, in one hour there 

 are consumed 1080 inches, and in 24 hours, 25,920 cubic inches or 15 cubic 

 feet, weighing 18 oz. To obtain this quantity, 300 cubic feet of air are 

 necessary. 



The quantity of oxygen consumed daily is subject to considerable varia- 

 tion. It is increased by exercise, digestion and lowered temperature, and 

 decreased by the opposite conditions. 



The quantity of carbonic acid exhaled in 24 hours varies greatly. It 

 can be estimated in the same way. Assuming that an individual exhales 

 93 ~f- cubic inch at each respiration, in one hour there are eliminated 1008 

 cubic inches, and in 24 hours, 24.192 cubic inches or 14 cubic feet, contain- 

 ing 7 ozs. of pure carbon. 



The exhalation of carbonic acid is increased by muscular exercise ; 

 nitrogenous food ; tea, coffee and rice; age, and by muscular development ; 

 decreased by a lowering of temperature ; repose ; gin and brandy, and a. 

 dry condition of the air. 



As there is always more oxygen consumed than carbonic acid exhaled, 

 and as oxygen unites with carbon to form an equal volume of carbonic acid, 

 it is evident that a certain quantity of oxygen disappears within the body. 

 In all probability it unites with the sulphur hydrogen of the food to form 

 water. 



